Thermoplastic resins such as thermoplastic elastomers, olefin polymers, vinyl polymers, and engineering plastics are excellent in physical properties, moldability, surface characteristics, and the like. Thus, they can be processed into a mass, a sheet, a film or another shape according to the applications and are used in many fields such as automobiles, home electric appliances, electronics, buildings, and sundries. In order to provide a product having a desired shape or having upgraded performance or various functions, a plurality of molded articles of these resins are bonded together and combined. In particular, a method is widely used, which involves stacking a covering material or decorating sheet excellent in surface characteristics, weather resistance, and decorativeness on the outer layer of a resin molded article that is excellent in mechanical properties, as a base material. Such laminates are commonly used in automobile interiors, house interiors, housings of household electrical appliances, and the like. These laminates, however, generally have poor adhesion between the layers. Hence, in many cases, an adhesive layer is provided between layers before lamination. As the adhesive, solvent based adhesives and hot melt adhesives are used. However, solvent based adhesives are disadvantageous in that they tend to cause uneven coating and that they have harmful effects on environment and hygiene due to the use of organic solvents. Therefore, there is a need for hot melt adhesives which are easy to use and excellent in adhesion strength.
Examples of such hot melt adhesives that have been proposed include those which contain at least one base polymer selected from the group consisting of ethylene copolymers, styrene block copolymers, and olefin (co)polymers, together with a tackifier resin and a crystalline compound containing a polar group (Patent Literature 1), those which contain an amorphous poly-α-olefin, a tackifier resin, and a polypropylene wax as essential components (Patent Literature 2), those which are obtained by adding a tackifier resin component and a liquid plasticizer such as process oil to a styrene-ethylene-propylene-styrene block copolymer rubber or a styrene-butadiene-styrene block copolymer rubber (Patent Literatures 3 and 4), those which are obtained by mixing a modified polyolefin and a tackifier (Patent Literature 5), those which are obtained by mixing a styrene block copolymer and an acid modified wax (Patent Literature 6), those which are obtained by mixing an acid-modified polypropylene and an acid modified styrene block copolymer (Patent Literature 7), and those which are obtained by mixing a styrene block copolymer, a tackifier, and an ethylene polymer (Patent Literatures 8, 9, and 10).
Though the above-mentioned adhesives, which contain a styrene block copolymer or a polyolefin as a base polymer, generally have certain adhesion strength to polyolefins, they have poor adhesion to polar resins such as acrylic resins and polycarbonate resins. These hot melt adhesives actually cannot be used to bond molded articles of different thermoplastic resins together because these molded articles usually have different polarities from each other. If bonding is performed under pressure and heat at a relatively high temperature, the adhesion to polar resins can be enhanced in some cases. However, in the case of preparation of laminates for something requiring aesthetic quality, such as automobile interiors, house interiors, and housings of household electrical appliances, such a bonding process causes problems of damage to molded members reducing aesthetic quality. An adhesive having a reduced active temperature can be provided by using a base polymer having a low softening point and melting point for an adhesive. However, such a hot melt adhesive is less likely to have practical heat resistance at about 100° C. If the adhesive is used to bond a deep-drawn molded article such as those for automobile interiors as an adherend, it causes appearance problems exemplified by lateral slip of the covering material due to the expansion and contraction of the cover in a high temperature atmosphere. Thus, this adhesive is impractical. Meanwhile, if a polyamide, a polyester, or the like is used as an adhesive component, the adhesive itself can have heat resistance; however, it has insufficient adhesion to low-polarity resins such as polyolefins.